1. Field of the Invention
The present invention relates to a color image pickup apparatus having an image pickup element constituted by a plurality of two-dimensionally arranged pixels.
2. Related Background Art
With the recent advances in the semiconductor technology, digital signal processing has become a main trend in the field of video signal processing. This is because digital signal processing is more advantageous than analog signal processing in terms of the integration and definition of ICs, the easiness of various adjustments, S/N ratio, and the like.
For this reason, image pickup elements of various schemes adapted for digital signal processing have been developed because they have the advantages of small size, light weight, high reliability, and the like. Of these image pickup elements, CCD type image pickup elements having the color filter arrangements shown in FIGS. 1 and 2 are most widely used nowadays as image pickup elements for video cameras and electronic still cameras in consideration of resolution, S/N ratio, easiness of signal processing, and the like.
When frame photography of a still image is to be performed by using an electronic still camera or the like, an image pickup element having the color filter arrangement shown in FIG. 1, in which color filters of different colors are repeatedly arranged at a two-pixel period in the horizontal direction, must be used for the following reason. A pixel signal formed by the image pickup element is read out alternately from an odd field and an even field, and identical outputs must be obtained from the two fields.
Referring to FIG. 1, the image pickup element has a pitch Ph in the horizontal direction and a pitch Pv in the vertical direction.
The image pickup element shown in FIG. 2 has a color filter arrangement different from the arrangement in which color filters of different colors are repeatedly arranged at a two-pixel period in the horizontal direction.
This image pickup element has a color filter arrangement in which color filters of different colors are repeatedly arranged at a two-pixel period in the horizontal direction. This filter arrangement is constituted by lines on which magenta (Mg) and green (Gr) filters are alternately arranged, and lines on which cyan (Cy) and yellow (Ye) filters are alternately arranged. The Mg and Gr filters are arranged in the form of an offset sampling array, whereas the Cy and Ye filters are arranged in the form of a square sampling array. Note that some image pickup element has a color filter arrangement in which Mg and Gr filters are arranged in the form of a square sampling array, and Cy and Ye filters are arranged in the form of an offset sampling array.
When signals are to be read out from the image pickup element shown in FIG. 2, a method called a mixed pixel read (or a field read) method is generally used. In this method, photoelectrically converted signal charges are read out, two lines at a time, in a mixed state. According to the method, signal charges are read out in the order of An.sub.n-1, A.sub.n, A.sub.n+1, . . . in the A field, and in the order of Bn.sub.n-1, B.sub.n, B.sub.n+1, . . . in the B field.
As a result, pixel signals (Mg+Cy) and (Ye and Gr), and (Cy+Gr) and (Mg+Ye) are alternately output from the image pickup element. For the sake of convenience, these four signals are respectively represented by
C.sub.1 =Mg+Cy PA1 C.sub.2 =Ye+Gr PA1 C.sub.3 =Cy+Gr PA1 C.sub.4 =Mg+Ye PA1 an image pickup element having four types of color filters which are arranged to form an offset sampling structure in which color filters of different colors are repeatedly arranged at a two-pixel period in a horizontal direction, and color filters of different colors are repeatedly arranged at a two-pixel period in a vertical direction while being offset in the horizontal direction by one pixel, and signal formation means for forming R, G, and B signals from the outputs from the four types of color filters by performing a matrix operation using a 3.times.4 matrix designed such that a sum of coefficients of two arbitrary columns in every row is equal to a sum of coefficients of remaining two columns.
In this case, when these four signals are obtained, a group of spatial barycenter positions can be considered as the sampling structure shown in FIG. 3, which is based on the signals C.sub.1, C.sub.2, C.sub.3, and C.sub.4. Therefore, in the mixed pixel read mode, the image pickup elements shown in FIGS. 29A and 29B are equivalent to an image pickup element having a color filter arrangement in which color filters of different colors are repeatedly arranged at a two-pixel period in the horizontal direction. A color image is obtained by forming a luminance signal and a color signal for each field on the basis of these pixel signals.
However, when the image pickup elements having the color filter arrangements shown in FIGS. 1 and 3 are to be used, in order to extract luminance and color signals, alternate line data (e.g., first and third line data in an odd field, and second and fourth line data in an even field) must be used.
With this arrangement, since data at two points spatially separated from each other in the vertical direction are added together, the correlation in the vertical direction deteriorates. As a result, the resolution in the vertical direction decreases, and the generation amount of pseudo color (color moire) increases.
Especially when photography of a still image is to be performed by using an electronic still camera, since only an image of one field can be obtained by the mixed pixel read method, a high-resolution image, i.e., a high-resolution frame image, cannot be obtained.
In general, the generation of this color moire can be suppressed by an optical low-pass filter placed in front of the image pickup element. On the other hand, the use of this optical low-pass filter leads to a decrease in the resolution in the horizontal direction. For this reason, in a conventional video camera, an optical low-pass filter must be designed to determine image quality in consideration of the trade-off between the resolution and the color moire caused by color carriers.
Recently, in order to improve image quality, a two-plate scheme color image pickup apparatus using two image pickup elements and a single plate scheme color image pickup apparatus using an image pickup element designed to ensure image quality as high as that of the two-plate scheme color image pickup apparatus have been used. Even these color image pickup apparatuses are not free from the above-described problems, i.e., the occurrence of color moire and the inability to obtain sufficient resolution.